Analysis of Arrester Energy for 132kV Overhead Transmission Line due to Back Flashover and Shielding Failure

Transcription

1 nalysis of rrester Energy for 132kV Overhead ransmission Line due to Back Flashover and Shielding Failure Nor Hidayah Nor Hassan 1,a, b. Halim bu Bakar 2,b, Hazlie Mokhlis 1, Hazlee zil Illias 1 1 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia 2 UM Power Energy Dedicated dvanced Center (UMPEDC), Level 4, Wisma R&D, University of Malaya, Jalan Pantai Baharu, Kuala Lumpur, Malaysia a hidayahassan@siswa.um.edu.my, b a.halim@um.edu.my bstract his paper presents the analysis of lightning arrester energy due to back flashover and shielding failure phenomena in a 132kV transmission line in Malaysia. he transmission line, towers and surge arresters were modeled using PSCD/EMDC software. he model has been used to simulate the discharged energy from lightning arresters that were installed on the tower in the event of back flashover and shielding failure. he arrester was modeled based on the IEEE frequency dependent model. Comparison between the simulation results and values calculated theoretically was performed to validate the model that has been developed. he results show that both are in reasonable agreement with each other. he maximum calculated and simulated energy discharged by the arrester is found to be less than 5.1 kj/kv, which is the rating of arresters installed in the actual 132kV transmission lines. Keywords-rrester; Back Flashover; Shielding Failure; Bergeron model; PSCD/EMDC I. INRODUCION Generally, the use of line arrester is to decrease or eliminate lightning flashover on transmission and distribution lines [1]. he purpose of line arrester installation in transmission line system is to improve the performance of overhead lines with poor shielding or with very high tower footing impedance [2]. rresters avoid lightning flashovers since transmission lines insulation voltage is higher than the residual voltage developed across the arresters, either due to back flashover or shielding failure. However, the arresters have to withstand the energy discharged by the lightning stroke. Shielding failure occurs when lightning strikes less than or equal to 20 k bypass the overhead ground wires [3]. It is always designed such that overhead ground wires are located at a position which provides the least shielding failure. hus, the majority of the lightning will terminate on the ground wires and build up a voltage across the line insulation. Back flashover will occur when these voltages exceed the line critical flashover (CFO). his paper presents the application of PSCD software to estimate the arrester energy due to shielding failure (SF) and back flashover (BFO) phenomena. he ranges of the current stroke used in the simulation, which contributes to both phenomena, are stated in [3]. he CIGRE simulation method for 132kV has been developed and the arrester was based on the frequency dependent model, which is represented with IEEE two sections of nonlinear resistance. Since the Maximum Continuous Operating Voltage (MCOV) of 132 kv transmission line is 97.2 kv, RVLQD - Class 2 type of arrester has been chosen because the rated voltage is between 3kV to 198kV as in the datasheet of porcelain type surge arrester from oshiba. II. MODELLING he overhead lines are represented by multi-phase model, which consider the distributed nature of the line parameters due to the range of frequencies involved. Phase conductors and shield wires are modeled in detail between the towers. PSCD/EMDC was used to model transmission line, towers and surge arresters. PSCD/EMDC was used because it offers real time analysis simulation. his software allows user to construct complex nonlinear power system models that combine three main components of a power system, power electronics and control circuits into one using more than 280 flexible components in the master library. Furthermore, PSCD/EMDC is suitable for simulating time domain instantaneous responses or electromagnetic transients for both electrical and control systems.. ransmission Line and ower Model ransmission line is modelled based on standard single circuit line geometry drawings and conductor data of a typical 132kV line. he transmission towers are represented geometrically similar to that of the single-storey lattice tower as shown in Figure 1. he lowest conductor from the ground is m and the span length of the transmission line is 300m. Line geometry for the tower configuration is shown in Figure 2. he surge propagation velocity is assumed equal to 85% of the speed of light [4]. here are several formulae to calculate the surge impedance of the tower. s a basis, (1), which is for waist tower shape [5] and recommended by IEEE and CIGRE [6] is used, Z t = 60 ln [cot {0.5 tan -1 (r avg / h)}] (1) /12/$ IEEE 683

2 r avg r h + r h h r h ravg + = (2) = equivalent radius of the tower represented by a truncated cone h = tower height, m h 1 = tower height from midsection to top, m h 2 = tower height from base to midsection, m r 1, r 2, r 3 = tower top, midsection and base radii, m 10 [pf] 10 [pf] 10 [pf] G1 B C 1s1 1s4 1s7 1s10 1s3 1s6 1s9 1s12 10[ohm] 1s2 G2 Figure 3. PSCD tower model Figure kv tower dimension B. Insulator String Model he insulator is modeled as a stray capacitor (C) connected in parallel with a voltage controlled switch (S) as shown in Figure 4. he string which consists of glass insulators, provides an equivalent capacitance used in the model. Insulator supports the conductor by providing mechanical support that depends on its normal operating and transient voltage. he voltage withstand capability of the insulator is calculated using 710 V o 0.9(400 + ) d 0.75 t = (3) V o = flashover voltage, kv t = time elapsed after lightning stroke, µs [8, 9] d = length of gap between arc horn, m Crossarm C S Conductor Figure 4 Insulator string flashover model Figure kV tower configuration Five transmission towers were modelled as single conductor distributed parameter line (Bergeron model travelling wave) segments of transmission lines in PSCD, as shown in Figure 3. Since the line parameters are constant at the chosen frequency when the Bergeron model is used, the user may select the R, L and C values. Line termination at each side of the model is necessary to avoid any reflection that might affect the simulated overvoltages around the point of impact [7]. C. Line rrester Model he line arrester characteristics used in the simulation are shown in able 1. BLE I. RRESER CHRCERISICS Nominal voltage(kv) 120 MCOV(kV rms) 97.2 Voltage(kV) for 10 k, 8/20μs 330 Energy absorption 4.5 Length of arrester column(m) No. of parallel column of disks 1 684

3 he non-linear characteristic of the line arrester is modeled as recommended by the IEEE W.G , which is metal oxide surge arrester [10]. IEEE line arrester model has been chosen because the oshiba surge arrester uses non linear resistor metal oxide elements as the main component. he frequency-dependent model consists of two non-linear resistors, 0 and 1, which are separated by an R-L filter, as shown in Figure 5. Figure 6 shows the V-I characteristics of 0 and 1 obtained from 8/20 µs impulse data, which is supplied by the manufacturer. III. LIGHNING he lightning source is modeled based on the IEC triangular wave shape, as shown in Figure 7 [11]. he lightning stroke is modeled by a current source in parallel with a lightning - path impedance, as shown in Figure 8. he lightning-path impedance is represented as a parallel resistance of 400 Ω [12]. peak current source of different magnitudes has been used to investigate the effects of shielding failure (SF) and back flashover (BFO) phenomena on the arrester discharge energy. able 2 shows the injected single stroke current for simulation of SF and BFO. I I peak t r = rise time t h = tail/half time I peak /2 Figure 5: IEEE frequency-dependent model line arrester Voltage, V (p.u) Current, I (k) 0 1 t r t h t (μs) Figure 7: Recommended IEC triangular wave shape [11] 31 Istroke IME * 400[ohm] able Figure 6: V-I non-linear characteristic for 0 and 1 he initial parameters of the resistor and inductor are calculated based on the estimated height of the arrester and the number of parallel columns of metal-oxide disks using [10] L 15d / n R 65d / n L o 0.2d / n 1 = (µh) (4) 1 = (Ω) (5) = (µh) (6) R o 100d / C 100n / n d = (Ω) (7) = (pf) (8) d= estimated height of the arrester (as in data sheet), m n= number of parallel columns of metal oxide in the arrester he values of 0, 1 and L 1 have to be adjusted so that the discharge voltages between theory and experiment have a good match. Figure 8: Lightning source model consisting of a current source and lightning path impedance BLE II. INJECED CURREN FOR SIMULION Injected Current, I (k) BFO SF ransmission shielding failure and back flashover were simulated by injecting single stroke currents to a line phase conductor or ground wire of the third tower. he time to rise, t r and time to half, t f, are chosen as 8μs and 20μs. 685

4 IV. RRESER ENERGY. Stroke to ower or Ground Wire he energy discharged by the line arrester, W, during back flashover can be estimated using [1] i e τ = arrester current, W = i e τ (9) = arrester discharge voltage, V = time constant, s he time constant of the arrester current, τ, is estimated by Z g R i s Z g τ = s (10) R = ground wire impedance, Ω = footing resistance, Ω i = span length divided by the velocity of light, s B. Stroke to Phase Conductor Lightning that struck at the phase conductor will give a different estimation of arrester energy discharged from lightning that struck at ground wire. he energy discharged by the surge arrester, W, during shielding failure can be expressed as an integral form of product of arrester current and discharge voltage [1]. W i = e dt 0 (11) he relation between the arrester current and discharge voltage can be represented by [13] α i = k( e ) (12) Combining both equation (11) and (12), simple arrester discharge energy equation can be express by K1Ie 1τ 1 W = (13) α where E 1 are the discharge voltage for current of K I I and α is -t / τ 1. Detailed derivation of both equations is explained in [1].. Stroke to ower or Ground Wire he stroke current strikes directly to ground wire or tower, creating back flashover phenomena. Figure 9 shows the voltage across the arrester when a lightning current of 20 k, 8/20 μs was injected on the third tower. y V m 0.10m 0.15m 0.20m 0.25m 0.30m Figure 9: Voltage across arrester for 20 k stroke to the ground wire he energy dissipated by the frequency dependent model, which consists of two non-linear resistors, 0 and 1 are shown in Figures 10 and 11. he total of these energies discharged by the line arrester is shown in Figure 12. Figure 10: 0 energy waveform for 20 k stroke to the ground wire Figure 11: 1 energy waveform for 20 k stroke to the ground wire V. RESULS ND DISCUSSION In practice, the 132kV transmission lines are equipped with 5.1kJ/kV energy of surge arrester. hus, comparisons between the calculation and simulation was performed for: a) back flashover for stroke current range of 20 k to 200 k b) shielding failure for stroke current range of 0 k to 20 k Over 50% of the lightning strokes contain more than one stroke, which is also known as multiple strokes lightning (MSL) [14]. However, in this work, single stroke lightning (SSL) current magnitude was only considered. Figure 12: Simulated energy discharged by the line arrester for 20 k stroke to the ground wire 686

5 able 3 and Figure 13 show comparison between the calculated energy values using (9) and the values obtained from simulation results of stroke currents between 20 k to 200 k. It was found that the simulated energy is slightly different than the calculated energy. his might due to the transmission line parameter used for tower models in the simulation are inaccurate. he energy discharged by the non-linear element of 0 and 1 obtained from the simulation when the peak current magnitude was injected to the conductor is shown in Figures 15 and 16. BLE III. Stroke Current (k) ENERGY OF RRESER DURING BCK FLSHOVER Calculated Energy Simulated Energy Figure 15: 0 energy waveform for 20 k stroke to phase conductor Figure 16: 1 energy waveform for 20 k stroke to phase conductor Figure 13: Comparison of arrester energy during back flashover Both 0 and 1 do not share the discharge energy equally because of the inductance between the elements. he sum of the two energy results in the total energy absorption of the line arrester due to shielding failure is shown in Figure 17. B. Stroke to Phase Conductor direct lightning stroke to phase conductor may result in a shielding failure. his occurs when lightning currents of less than or equal to 20k bypass the overhead shield wire. For this case, only SSL current magnitudes of 5k to 20k were simulated because shielding failures tend to occur between these values. he designed line arrester was placed at the top phase 1 of the third tower and different SSL currents were injected at the top phase conductor. he waveform of the discharge voltage across the phase 1 when a 20k lightning stroke terminates at the top conductor is shown in Figure 14. Figure 14: Voltage across surge arrester for 20 k stroke to phase conductor Figure 17: Simulated energy discharged by the line arrester for shielding failures o calculate the arrester energy analytically, the stroke current is assumed to have exponential tail decay. he time to half value of the stroke current is chosen as 20μs, hence the tail time constant, τ, is 29μs. ssuming the arrester discharges the entire stroke current, the energy dissipated can be calculated from (10) and (11). Figure 18 shows comparison between the calculated and simulated results of the arrester discharge energy in kj due to SSL striking the top phase conductor. able 4 summarizes the results of the obtained total energy dissipated by the arrester in kj/kv of MCOV for different peak current magnitudes. Increase in the lightning current results in a higher energy discharged by the arrester. 687

6 Figure 18: Comparison of arrester energy during shielding failure BLE IV. Stroke Current (k) ENERGY OF RRESER DURING SHIELDING FILURE Calculated Energy Simulated Energy C. Effect of footing resistance he effect of the footing impedance was analyzed by assuming a simple linear resistance for the its model. able V shows the energy discharged through the arrester installed at phase of the test line as a function of the tower footing resistance for both back flashover and shielding failure. Note that, as the footing resistance increase, energy discharged for lightning that strike to the ground wire also increase. For lightning strikes at phase conductor, arrester energy remains the same as the footing resistance increase. herefore, any changes to the footing resistance does not change the energy of arrester for shielding failure phenomena. his situation differs for back flashover phenomena as the energy discharged increase if the footing resistance increase. BLE V. ENERGY OF RRESER FOR DIFFEREN FOOING RESISNCE Energy, kj/kv Rf, Curent 200 k at ground Current 18 k at phase Ω wire (Back flashover) (Shielding Failure) VI. CONCLUSION he work in this paper has investigated the capability of the arresters installed in 132 kv transmission lines in Malaysia, in withstanding single stroke lightning discharged energy caused by shielding failure and back flashover phenomena. he arrester energy discharged due to shielding failure and back flashover was calculated using analytical method and simulation using PSCD/EMDC. Comparison between the simulation results and values calculated theoretically shows that both results are within reasonable agreement to each other. he effect of footing resistance to the energy discharged also been covered in this paper. Only back flashover phenomena will increase the energy discharged as the footing resistance increase. he rating of the arresters installed in the actual 132kV transmission lines, which is 5.1 kj/kv, has been found capable of handling the maximum energy discharged by the designed arrester. Future work will consider other current impulse wave shape and multiple strokes lightning to study the behavior of arrester energy. CKNOWLEDGMEN he author thanks the University of Malaya for supporting this work through the IPPP research grant (Grant no: PS ). REFERENCES [1]. R. Hillman, Insulation Coordination for Power Systems, Marcel Dekker, [2] CIGRE WG 33.11, pplication of Metal Oxide Surge rresters to Overhead Lines, Electra, October [3] C.. Nucci, Survey on Cigré and IEEE Procedures for the Estimation of the Lightning Performance of Overhead ransmission and Distribution Lines, in sia-pacific International Symposium on Electromagnetic Compatibility (PEMC), Beijing, pp , [4] IEEE Working Group, Simplified Method for Estimating Lightning Performance of ransmission Lines, IEEE rans. Power pp. Syst., vol. PS-104, no. 4, pp , pr [5] W.. Chisholm, Y.L. Chow, K.D. Srivastava, ravel ime of ransmission owers, IEEE rans. on Power pparatus and Systems, vol. 104, no. 10, pp , October [6] CIGRE WG 33.01, Guide to Procedures for Estimating the Lightning Performance of ransmission Lines, CIGRE Brochure 63, [7] Juan. Martinez-Velasco, Castro-randa; Modeling of Overhead ransmission Lines for Lightning Studies, IPS 05 in Motreal, Canada No. IPS05 190, June 19-23, [8] IEEE Standard, IEEE Guide for Improving the Lightning Performance of ransmission Lines, IEEE Std pp [9] D. Caulker,. Hussein, Z.. Malek, S. Yusof, Shielding Failure nalysis of 132 kv ransmission Line Shielded by Surge rresters ssociated with Multiple Strokes Lightning, International Conference on Electrical and Computer Engineering (ICECE), 2010, vol., no., pp , Dec [10] IEEE WG , Modelling of Metal Oxide Surge rresters, IEEE rans. On Power Delivery, vol. 7, pp , Jan [11].H.. Bakar, H. Mokhlis,.L. Lim, W.P. Hew, Lightning Over voltage performance of 132kV GIS Substation in Malaysia, International Conference on Power System echnology (POWERCON), 2010, vol., no., pp.1-7, Oct [12]. metani,. Kawamura, Method of a Lightning Surge nalysis Recommended in Japan Using EMP, IEEE rans. on Power Delivery, Vol. 20, No. 2, pp , pril [13]. E. McDermott, D. E. Parrish, and D. B. Miller, Lightning Protection and Design Workstation Seminar Notes, EPRI R , Sep [14] PSCD/EMDC Manual, Introduction to PSCD/EMDC V3, Manitoba HVDC Research Centre Inc